1. Technical Field
The present invention refers to a method and an apparatus for suppressing the presence of multipath propagation in a broadcast signal.
The present invention moreover refers to a radio receiver using said apparatus.
The present invention refers to a method and an apparatus for detecting adjacent channel interference in a modulated digital signal.
The present invention moreover refers to a method and apparatus for suppressing adjacent channel interference in a modulated digital signal.
The present invention also refers to a radio receiver using said apparatus.
2. Description of the Related Art
In terrestrial broadcasting systems, the signals emitted from a transmission antenna often arrive to a reception antenna not only through a direct transmission antenna to reception antenna path, but also through many other paths.
Therefore, the reception antenna does not receive a single signal, but many signals.
This is due to the fact that the transmitted signal follows several different paths due to the reflections to which such signal is subjected. Indeed, during their propagation the transmitted signals are subjected to reflections which are caused by objects present in the broadcasting zone.
The signals which arise from such reflections are called multipath signals.
In reality, the signals received by the reception antenna reach in different times, are not in phase and do not have the same intensity, since the objects present in the broadcasting zone can be fixed in time and space, such as buildings, and/or can be movable, such as airplanes.
The signals, moreover, are combined with each other when they reach the reception antenna in a constructive or destructive manner, and the overall resulting effect is that the level of the input signal at the receiver varies considerably.
Essentially, the signal reaching the receiver is a combination of the source signal and the replicas of such source signal, each having different courses, delays and displacements.
There is hence the great need for suppressing the multipath propagation in the broadcast signal.
The effects of multipath propagation, therefore, must be at least theoretically suppressed or at least reduced during the demodulation process so to permit a correct reconstruction of the broadcast signal.
For example, the document US 2004/0042571 describes a system for suppressing the presence of multipath propagation in a broadcast signal.
Such system, even if certain advantageous features are present, nevertheless requires numerous circuit components and the implementation of specific software which is particular burdensome from the standpoint of computational complexity, with the consequence of having to employ high memory quantities.
Further examples of systems for suppressing the presence of multipath propagation in a broadcast signal are described in documents US 2005/0276365 and U.S. Pat. No. 7,221,925.
The problem of adjacent channel interference in broadcast signals is known, particularly in AM/FM signals. Adjacent channel interference is due to the presence of channels with high field intensity adjacent to the selected channel which one wishes to receive. In the present case, the radio waves of the adjacent channel are found to interfere with the radio waves of the desired broadcast channel, causing disturbances and/or distortions in the reception of the desired channel signal by an AM/FM radio receiver.
At the state of the art, various methods and apparatuses are known for detecting and reducing/suppressing the adjacent channel interference in a broadcast AM/FM signal.
In FM mode, the prior art provides the analysis of the radio signal in complex notation, centered on an intermediate frequency IF; the greater the detected displacement of the oscillation frequency of the carrier signal component, the greater the interference contribution of the undesired adjacent FM channel within the desired FM channel.
In the case of a signal of AM type, a prior art example provides, as in the case of an FM signal, the analysis of the broadcast signal DSB with dual side bands, it too centered on the intermediate frequency IF and a subsequent choice of the “cleanest” side band for the demodulation of the AM signal.
Other techniques require high performance hardware circuitry with a consequent increase in the costs of the apparatus for detecting and suppressing the adjacent channel interference, and consequently, in the overall cost of the AM/FM radio receiver in which such apparatus is incorporated.
Examples of additional techniques for detecting and reducing or suppressing the adjacent channel interference are described in the documents U.S. Pat. No. 6,430,724, US-2003/0207669 and WO-2004/047322.
Such documents describe methods and apparatuses which require numerous circuit components and complex processing algorithms for detecting and suppressing the adjacent channel interference in the received signal. Moreover, the methods and apparatuses therein described require a conversion of the received signal from the frequency of the selected carrier to an intermediate frequency IF, and hence the use of external mixers and filters for the intermediate frequency whose cost substantially impacts on the overall cost of the receiver.